The use of elastomeric material in compression springs is commonly known. As discussed in U.S. Pat. Nos. 4,198,037 and 4,566,678, issued on Apr. 15, 1980 and Jan. 28, 1986, respectively to the assignee of the present application, a useful thermoplastic elastomer is a copolyester polymer elastomer such as sold by E.I. duPont de Nemours & Co. of Wilmington, Del., under the trademark HYTREL. As explained in those patents, HYTREL is made from three ingredients, namely, dimethyl terephthalate; polyglycols such as polytetramethylene ether glycol, polyethylene ether glycol or polypropylene ether glycol; and short chain diols like butanediol and ethylene glycol.
Ordinarily, this type of polymer elastomer material has inherent physical properties that make it unsuitable for use as a compression spring. However, the above-mentioned patents describe a method by which the copolyester polymer elastomer material can be converted into a compression spring material. Generally, that method includes the application of a compressive force to a body of material which compresses the body in an axial direction to an extent greater than 30% of its previous axial length. The '678 patent focuses on an improvement wherein a hollow spring is formed to overcome the compression set characteristics of HYTREL. The provision of a central core or opening has been found to enhance the characteristics of the spring and enlarge the scope of applications where such a spring can be utilized. By varying the shape and size of the hollow core, elastomeric springs with various spring rates can be produced to accommodate the loads and deflections needed in particular spring applications.
Notwithstanding these improvements, there remains a need to enhance the characteristics of elastomeric compression springs by increasing energy absorption while maintaining desirable functional characteristics. Typically, elastomeric springs provide force/deflection curves which vary depending on the material and configuration of the spring. As used herein, the term "stiff" refers to those portions of the force/deflection curve that have a relatively sharp upward slope, which indicates a relatively high spring rate. Whether the curve slopes sharply upward at a constant or generally rising rate, such a slope is characteristic of progressively increasing resistance to compressive deformation. The term "soft" refers to those portions of the force/deflection curve that have a relatively gradual slope, and is characteristic of relatively lower spring rate, whether the slope is constant, increasing or decreasing.